1. Field of the Invention
The present invention relates to a wireless IC device for use in, for example, a near field wireless communication device, such as a radio frequency identification (RFID) tag, and also relates to a molded resin article including the wireless IC device and a method for manufacturing a coil antenna.
2. Description of the Related Art
Although HF-band RFID tags are generally card-sized, small RFID tags that occupy a small area are also needed for the purpose of, for example, merchandise management. Examples of small HF-band RFID tags include RFID tags having shapes described in Japanese Unexamined Patent Application Publication No. 2007-102348 and International Publication No. 2011/108340. These small RFID tags are formed by using a sheet laminating method in which an RFIC chip is mounted on a multilayer substrate having a layered type coil antenna disposed therein.
Through the development of the above-described small RFID tags, the inventor has found that the RFID tags described in Japanese Unexamined Patent Application Publication No. 2007-102348 and International Publication No. 2011/108340 have the following problems.
In the RFID tags described in Japanese Unexamined Patent Application Publication No. 2007-102348 and International Publication No. 2011/108340, an RFIC chip is disposed on the central axis of a coil antenna or in a coil opening of the coil antenna. Therefore, the electrodes (land patterns) used to mount the RFIC chip intersect the winding axis of the coil antenna. As a result, the RFIC chip and the electrodes used to mount the RFIC chip tend to impede the formation of a magnetic field by the coil antenna. Although the formation of the magnetic field is not easily impeded when the RFIC chip is disposed at an outer side portion of the coil opening, the occupied area increases in such a case.
Since the RFIC chip is disposed on the central axis of the coil antenna or in the coil opening of the coil antenna, there is a possibility that various circuits included in the RFIC chip will malfunction due to the influence of the magnetic field. In addition, when the coil antenna transmits and receives a weak magnetic field, there is a possibility that the performance (sensitivity) of the coil antenna will be reduced due to noise generated by a digital circuit unit included in the RFIC chip.
When, in particular, the coil antenna is manufactured by a sheet laminating method, it is necessary to consider the displacements of the sheets (lamination position accuracy) and flatness of the multilayer body; therefore, there is a limit to how much the number of laminated sheets can be increased and how thick the coil pattern can be made. For this reason, the achievable inductance value is limited, and it is particularly difficult to realize a coil antenna having a low direct current resistance (DCR). It is possible to form a coil pattern having a coil winding axis that extends in a planar direction of the sheets. However, in such a case, since the number of sheets that can be laminated is limited, it is difficult to increase the coil opening area and to obtain a coil antenna having a low direct current resistance.